The invention is based on a fuel injection pump for internal combustion engines.
In fuel injection pumps of this kind, used for Diesel engines, the injection duration is lengthened in idling and at low partial load for the sake of quieter idling, while at upper partial load or full load it is left unchanged.
In a known fuel injection pump of this kind (German Offenlegungsschrift 30 13 087), the relief conduit is connected for this purpose to a pump interior that can be made to communicate with the pump work chamber via a suction conduit and longitudinal grooves in the pump piston and is filled with fuel at feed pressure. A governor slide that slides on the pump piston in the pump work chamber controls a control bore that communicates with the pump work chamber, and thereby determines the end or onset of injection depending on the position of the control bore in the pump piston. The mouth of the relief conduit in the pump work chamber is guided via the pump piston and is covered by the governor slide in such a manner that the mouth is uncovered only once while the pump piston is executing a predetermined pre-stroke for generating the injection pressure. Once the mouth of the relief conduit is uncovered, part of the fuel in the pump work chamber flows back into the pump interior. The diversion of part of the fuel thus takes place only after the opening of the injection nozzle, that is, after the injection onset. At upper partial load or full load, the control slide is displaced into its blocking position, in which it blocks off the relief conduit, by the governor slide, or a control element engaging the governor slide. From then on, the injection duration is determined only by the pump piston stroke and the position of the governor slide.
With this kind of quiet-idling provision, it must be noted that typically there is a stroke difference of only 0.1 mm at maximum between the "end of supply, idling" position and the "end of supply, full load" position. This slight stroke difference is all that is available for the control path of the control slide. Given this small coincidence value and the high pressure of the fuel located in the pump work chamber, inconsiderable leakage problems may arise, which can be overcome only if the pump piston head is accurately calibrated with respect to the location of its control bore.
Furthermore, the "pre-stressing" of the entire high-pressure space until the relief conduit is uncovered is defined via a fixed ground-in dimension of the pump piston head. If the nozzle opening pressure changes, for instance, or if carbonization partially plugs the opening, the pre-stressing cannot be adapted to these changes. The tolerances of the governor slide and control bore also enter into this ground-in dimension of the pump piston head, however, and so re-machining of this dimension is often necessary. This makes mass production of the known injection pump problematic.